Over 40 million people are currently infected with HIV-1 and, considering the lack of a prophylactic vaccine in the foreseeable future, this number is expected to rise exponentially in many underdeveloped regions of the world. It is likely that humoral immunity will be a necessary component of vaccine-induced protection against HIV-1 infection; however, it has proven especially difficult to elicit broad and potent neutralizing antibodies (Nab) against the HIV-1 envelope (Env) glycoproteins. Several recent studies provide optimism that the viruses that are transmitted and establish infection in some settings pass through a genetic bottleneck that could be targeted to protect against HIV-1 infection: (i) newly transmitted subtype C viruses in Zambia have less glycosylated, more compact variable loop regions in Env and are more neutralization sensitive than the non-transmitted viruses from the chronically infected index case, (ii)newly transmitted subtype A viruses in Kenya have Envs with shorter V1V2 loop sequences and fewer N-linked glycosylation sites relative to the circulating population, and (iii)SIVsm viruses that establish infection in rhesus macaques (a non-natural host) have compact, less glycosylated V1V2 domains in Env compared to the variants present in the inoculum. The current proposal builds on the original finding described above that transmission of subtype C HIV-1 from a chronically infected partner appears to select FOR a virus with a compact Env that is neutralization sensitive, and AGAINST neutralization resistant viruses with large, heavily glycosylated Envs in the quasispecies of the index case. Our hypothesis is that this bottleneck produces a unique yet transient Env antigen that will induce antibodies upon immunization of guinea pigs that are able to neutralize the autologous virus and cross-neutralize other newly transmitted strains. We propose that the newly transmitted Envs will elicit antibodies to conserved neutralization epitopes that are not normally accessible, such as the coreceptor and CD4 binding domain, because exposure of these regions is important for transmission or outgrowth. By contrast, neutralization resistant Envs derived from the chronically infected index case will fail to induce antibodies that can neutralize the newly transmitted strains in our model. The Specific Aims are to (i) generate tropism-modified heterologous adenovirus type 5 (Ad5) vaccine vectors that express matched newly transmitted and chronic subtype C HIV-1 Envs for immunization of guinea pigs and (ii)compare the abilities of matched donor and recipient Envs to elicit neutralizing antibodies against a panel of autologous and heterologous subtype C Env pseudoviruses..